1. Field of the Invention
This invention relates to a data reception unit for use in a communication system such as a video communication system. In particular, the invention relates to a data reception unit having the ability to prevent deterioration in image quality resulting from data errors, drop outs, and other problems occurring during transmission.
2. Description of the Related Art
Communication systems known in the prior art include motion video transmission systems. Motion video systems include video telephony systems, video conferencing systems, and video on demand (VOD) systems. The task of developing international standards for motion video encoding is ongoing with the goal of developing systems for transmitting motion video. Representative international standards include ITU-T H.261 and H.263, as well as the ISO's international standards MPEG (Moving Pictures Experts Group) phases 1, 2, and 4.
Coded data comprises a series of codewords (hereinafter referred to as "bit stream"). For transmission, the bit stream is not transmitted as-is but is rather broken up into shorter data sequences. Specifically, the serial bit stream is broken up into data units (hereinafter termed "packets") of finite length for transmission. Each packet has a header and a payload; the header contains control information and the payload contains a coded data unit.
Data errors occurring during transmission of encoded motion video prevent the receiving unit from correctly decoding the coded data. Particularly where variable length encoding has been used, such errors not only prevent the particular codeword from being decoded, but also prevent correct decoding of other codewords coming after the codeword in question. Variable length encoding is widely employed in the international standards.
In motion video transmission systems employing variable length encoding, it is common practice to periodically insert synchronization codes into the coded data. Synchronization codes consist of unique bit sequences of a type certain not to appear in the encoded motion video data. In the event of a codeword error in received data, the decoder is able to resynchronize through detection of the synchronization code. Resynchronization allows the decoder to correctly decode subsequent codewords.
In ITU-T H.263, coded data frames each consist of a block that starts with a Picture Start Code (PSC), as well as a Group of Blocks (GOB). A PSC is inserted into the header for each frame. GOB refers to the blocks of the frame, and consists of a plurality of a macroblock (MB) lines. The MB is the unit for encoding. The header of macroblock contains a Group of Block Start Code (GBSC) that functions as a synchronization code.
The PSC has fixed word length and is represented by the 22 bit sequence "0000 0000 0000 0000 1 00000". The GBS also has fixed word length and is represented by the 17 bit sequence "0000 0000 0000 0000 1". The leading 17 bits of each of the two types of synchronization code have the same value. Accordingly, the leading 17 bits of the PSC and the leading 17 bits of the GBS can be viewed as a single synchronization code.
Even a single bit error will render the synchronization code undetectable. Accordingly, the synchronization code is not robust against transmission errors occurring with long codewords.